The present invention relates to a thin-film semiconductor device, in which a thin-film transistor and a capacitive element are provided on the same substrate, to an electro-optical device that uses such a thin-film semiconductor device as an electro-optical device substrate, and to an electronic apparatus having such an electro-optical device. More particularly, the present invention relates to a technology that can enhance the capacitance and breakdown voltage of a capacitive element.
When a thin-film transistor (hereinafter, referred to as a TFT) and a capacitive element are formed on the same substrate, a first electrode is formed using a semiconductor film formed on the same layer as an active layer of the TFT to be electrically conducted, a dielectric film is formed using an insulating film formed on the same layer as the gate insulating film, and a second electrode is formed using a conductive film formed on the same layer as the gate electrode. In this case, the TFT and the capacitive element can be formed with a small number of processes. This structure is widely used in an element substrate of a liquid crystal device (electro-optical device), in which the TFT is used as a nonlinear pixel-switching element, or in various thin-film semiconductor devices.
When the thickness of a dielectric film is small in the capacitive element, a large capacitance can be obtained. On the other hand, when the thickness of the gate insulating film is small in the TFT, the breakdown voltage may be reduced. Accordingly, a structure has been suggested in which, on the side of the capacitive element, the insulating film formed simultaneously with the gate insulating film is reduced in thickness so as to form the dielectric film (for example, see Japanese Unexamined Patent Application Publication No. 6-130413).
In the liquid crystal device, the area per pixel has significantly decreased as the demands for high-definition images have increased. For this reason, the liquid crystal capacitance gradually decreases, so that a capacitive element needs to have a large capacitance. However, when the dielectric film is reduced in thickness in the capacitive element in order to increase the capacitance, the breakdown voltage decreases, so that the yield or reliability of the liquid crystal device deteriorates.
The above-described problems can be solved by a method in which the dielectric film is reduced in thickness in the capacitive element in order to secure the breakdown voltage and an opposing area of first and second electrodes is expanded in order to increase the capacitance. However, in all semiconductor devices including the liquid crystal device, the expansion of an element formation area may be spatially restricted.